1. Field
This invention relates to displacement sensing devices for detecting the position of a movable element sensor used in process control systems and, more particularly, to an optical fiber transducer which uses two wavelengths of light for determining the position of the movable element.
2. Description of the Prior Art
A typical optical fiber displacement transducer includes an optical fiber cable connecting a light-source and light detector to a sensor. An optical signal generated by the source is transmitted through the cable to the sensor. The sensor in response to a physical variable that is desired to be measured modulates a characteristic of the optical signal in accordance with changes in the physical variable. The modulated signal is thereafter transmitted to the detector which converts that signal to a useful output representative of the magnitude of the physical variable.
Being affected by minute changes, the displacement transducer is subject to the problems of drift which usually arise because of instabilities and changes in the operation of the light-source and/or the detector. Furthermore, age may change the transmission properties of the optical cables and the couplers joining together those cables thereby introducing other sources of drift. As a result, even though transducer measurement sensitivity is high, these instabilities and changes limit the uses of these transducers to process control applications where moderate to high measurement accuracy and repeatability are not required.
These problems may be overcome to some degree by periodically recalibrating these transducers. But such recalibrations after installation will certainly add to the operational costs. Moreover, in process control applications where transmitters are installed at remote stations, periodic recalibration may not always be possible when needed because accessibility to these remote sensors is limited, such as by weather conditions.
In U.S. Pat. No. 4,249,076, issued to Bergstrom et al, an optical measuring device is disclosed wherein a portion of the modulated signal is used as a feedback control signal for insuring a constant level output from the light-source. However, this device still does not overcome the troubles that may arise from instabilities in the light detectors or in the optical cables. Furthermore, the modulated signal is divided and transmitted simultaneously through at least two optical cables to respective measurement detectors coupled to each cable. This arrangement requires that these two (or more) optical cables have matching optical properties and performances so that accurate measurements can be derived from the modulated signal produced by the sensor. The use of multiple parallel routes will increase the sources of drift that adversely affect transducer performance. Additionally, in applications requiring a sensor located at a remote station, the adverse effects of these sources of drift can increase as the lengths of the parallel routes are increased. Therefore, there is a need for improvement in fiber optic transducers, especially in those process control applications where there are moderate to high demands on accuracy and repeatability of measurements.